Gas turbine engines, such as those used to power modern aircraft or in industrial applications, are axial flow rotary machines. Gas turbine engines include a compressor for pressurizing a supply of air, a combustor for burning a hydrocarbon fuel in the presence of the pressurized air, and a turbine for extracting energy from the resultant combustion gases. Generally, the compressor, combustor and turbine are disposed about a central engine axis with the compressor disposed axially upstream of the combustor and the turbine disposed axially downstream of the combustor. Air drawn into the engine passes axially through the compressor into the combustor wherein fuel is combusted in the air to generate and accelerate combustion gases that pass through the turbine and out the exhaust nozzle of the gas turbine engine. The combustion gases turn the turbine, which turns a shaft in common with the compressor to drive the compressor.
The compressor of the gas turbine engine includes a rotor assembly and a stator assembly disposed coaxially about an axis of rotation. The rotor assembly includes a series of axially spaced rotor stages mounted to a rotor shaft structure. Each rotor stage includes an array of airfoils, termed rotor blades, extending outwardly from and at circumferentially spaced intervals about the rotor shaft structure. The stator assembly includes an outer stator case that coaxially circumscribes the rotor assembly and includes a plurality of stator vane stages disposed at axially spaced intervals such that a stage of rotor blades extends outwardly axially aft of each stage of stator vanes to terminate in close proximity to the outer stator case of the stator assembly.
Each stator vane stage includes a plurality of circumferentially spaced stator vanes supported from the outer stator cases and extending inwardly to an inner stator case circumscribing and in close proximity with the rotor shaft structure. A circumferentially extending inner airseal is mounted to the inboard surface of the inner stator case of each stage of stator vanes. The inboard surface of the inner airseal in cooperation with a projecting structure on the rotor structure, such as a knife edge seal element, establishes the air seal at each stage of stator vanes. In conventional practice, for the hotter stages of the compressor, the inner airseal is typically made of an abradable material, such as a porous metal fiber, brazed to a substrate surface on the inboard end of the stator vanes. The use of porous metal fiber materials seals brazed to the substrate surface in the hotter stages of the compressor is necessary due to the higher air temperatures to which the inner airseal is exposed. However, in the cooler stages of the compressor, the inner airseal is typically made of an abradable material, such as an elastomeric material, adhesively bonded to a substrate surface on the inboard end of the stator vanes.
In operation, as the rotor shaft structure rotates, the knife edge seal element will contact and even cut sealing grooves into the surface of the inner airseal to minimize air leakage. Over time in operation, the seal material becomes worn down and it becomes necessary to restore the inner airseal to insure the integrity of the air seal and maintain efficient operation of the gas turbine engine. In the repair of inner airseals, certain repair techniques applicable to one type of abradable material may not be applicable to another type of abradable material as the process of brazing and the process of adhesive bonding are mutually exclusive processes based on processing temperatures required for each type of operation.
The conventional practice for repair of damage or worn inner airseals made of elastomeric material is to remove all of the elastomeric material of the inner airseal under repair, including all of the remaining undamaged and unworn elastomeric material to expose the underlying surface of the inner case and usually around the opening in the inner case through which the vane tips extend. The removal of the elastomeric material is performed by machining or abrasive blasting and requires precise control to avoid damage to the retaining clips which engage the respective tip portions of the roots of the stator vanes that extend inboardly through the inner case and/or damage to the surface of the inner case and the roots of the stator vanes. Generally, it is necessary to replace the exposed retaining clips even if not damaged during the machining process per se. More importantly, the position and alignment of the stator vanes needs to be re-established relative to the case datum. Once the original elastomeric material has been completely removed and the underlying surface of the inner case and the exposed root portions of the stator vanes are thoroughly cleaned, a new elastomeric seal element is applied in the same manner as during original equipment manufacture. Consequently, the current repair practice requires precise machining and intricate preparation, which is time consuming and labor intensive. This is particularly true for aluminum cases, which require special processing to prepare the surface for bonding if the base alloy is exposed during removal of the old abradable airseal.